Abstract
Sucrose synthase (SuSy) is one key enzyme directly hydrolyzing sucrose to supply substrates for plant metabolism, and is considered to be a biomarker for plant sink strength. Improvement in plant sink strength could lead to enhanced plant growth and yield. Cultivated tomatoes are known to have a narrow genetic diversity, which hampers further breeding for novel and improved traits in new cultivars. In this study, we observed limited genetic variation in SuSy1, SuSy3 and SuSy4 in 53 accessions of cultivated tomato and landraces, but identified a wealth of genetic diversity in 32 accessions of related wild species. The variation in the deduced amino acid sequences was grouped into 23, 22, and 17 distinct haplotypes for SuSy1/3/4, respectively. Strikingly, all known substrate binding sites were highly conserved, as well as most of the phosphorylation sites except in SuSy1. Two SuSy1 and three SuSy3 protein variants were heterologously expressed to study the effect of the amino acid changes on enzyme kinetic properties, i.e. maximal sucrose hydrolyzing capacity (Vmax), affinity for sucrose (Km), and catalytic efficiency (Vmax/Km) at 25°C and 16°C. SuSy1-haplotype#3 containing phosphorylation site Ser-16 did not have an improvement in the kinetic properties compared to the reference SuSy1-haplotype#1 containing Arg-16. Meanwhile SuSy3-haplotype#9 from a wild accession, containing four amino acid changes S53A, S106I, E727D and K741E, showed an increase in Vmax/Km at 16°C compared to the reference SuSy3-haplotype#1. This study demonstrates that SuSy kinetic properties can be enhanced by exploiting natural variation, and the potential of this enzyme to improve sucrose metabolism and eventually sink strength in planta.
Highlights
In the plant kingdom, sucrose is the major product of photosynthesis
This study focused on exploring the natural genetic variation among the three SuSy1/3/4 isoforms using the available genome sequences of 84 accessions including cultivars, landraces, and wild species from The 150 Tomato Genome ReSequencing project [19], Exploring natural genetic variation in tomato sucrose synthases and one extra high-altitude wild species accession obtained from Tomato Genetics Resource Center in Davis, California (TGRC, http://tgrc.ucdavis.edu/)
The identified variations in SuSy1/3/4 of 85 accessions were distributed over all domains identified for AtSuSy1: cellular target domain (CTD), ENOD40 peptide-binding domain (EPBD), GT-B glycosyltransferase domain (GT-B), and C-terminal extension (C), except for SuSy4, which had no variation in the C domain (Tables 1–3)
Summary
The aim of this study was to find the SuSy protein variants among the landrace and wild accessions having improved kinetics at different temperatures compared to that of the control cultivated tomato
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